In That '70s Show, Mila Kunis' Jackie always called the other characters by their first names, and it wasn't just an empty quirk. Despite being the youngest character in the main cast of the show, Jackie's apparent need to assert dominance over others by appearing authoritative, made her feel like the group's mother figure. This, along with her familiarity through dating many of the boys in the group, informed her habit of calling everyone by their first names.
With the exception of Fez, whose real name is never revealed, the gang all refer to one another by last names, sometimes even with Donna (or Pinciotti, more appropriately). As a result, it is particularly noticeable when Jackie uses first names, painting her as the odd one out. But it is perhaps precisely for this reason that Jackie makes this distinction. As Kelso's girlfriend, she isn't really part of the group. She is friends with them by default and is younger and smaller than them, which informs her joking about Donna being so tall. She is viewed as a snob or a spoiled rich girl for the most part and she and Eric seem to clash the most.
Jackie's use of first names appears to be a method by which she distinguishes herself. There is something inherently motherly and formal about using a first name, so Jackie may have been trying to age herself up. Generally speaking, Jackie acts, in a way, as the matriarch of the group, which fits with her behavior choices but that's not the only reason she tends to avoid nicknames.
In contrast, Donna is portrayed as "one of the guys" or "the girl next door," while Jackie is confident and self-assured, demanding respect and attention from others, especially as the show goes on. Where some might deem her high-maintenance, her firm sense of boundaries and obvious emotional intelligence predicates her position as a leader. Jackie doesn't have to adhere to what everyone else is doing, and she revels in her unyielding sense of self. It is clear through her bold sense of fashion and refusal to be ignored that she yearns to rise above the pack, and calling her friends by their first names in defiance of their tradition is one of the many ways in which she accomplishes this.
There's also the issue of romance on That '70s Show. When someone is romantically involved, they often call their paramour by a different nickname or moniker than the rest of their friends. This can serve as a designation of affection, but also creates a sense of ownership. Jackie hides her insecurity and loneliness behind a veneer of conceit and selfishness, but she appears to crave closeness and a sense of belonging. When her father is jailed, her mother abandons her, making it clear that Jackie has an unsatisfying home life and therefore seeks intimacy with the company of boys. Through the series she dates Kelso, Hyde, and Fez, calling Kelso "Michael" and Hyde "Steven," emphasizing her special relationship or ties to them. Their friends are permitted to call them by their surnames, but Jackie sets herself apart as someone close enough to them to refer to them in a more exceptional way.
Though never portrayed as an intellectual, Jackie showed great acumen for manipulation and pot-stirring. Her election to use first names may have been a method by which she set the others just a little off-kilter, making them wonder how it was she perceived them. Jackie could be critical of others and sometimes downright mean, so the others likely felt a slight bit of insecurity when she didn't use their buddy names. The way in which she impacted or affected other people was of great importance to Jackie, so there is a chance her unique name-giving was simply an over-arching method to influence the emotions of others. Like a Disney villain, Jackie is aware of her powers and uses them unabashedly, making her one of That '70s Show's most distinctive and fascinating characters.